Support numerical transfer functions in readPixels()

Let's do this because:
(1) We can.
(2) Android and Chrome have asked for it.
(3) It will simplify the implementation of SkImage::makeColorSpace().

Bug: skia:
Change-Id: Ia3c322b8a58c79ad67cdebe744e0623bd59dcffd
Reviewed-on: https://skia-review.googlesource.com/15148
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Matt Sarett <msarett@google.com>
3 files changed